Labour still has a "mountain to climb" to win the next election, despite being
more than 10 points ahead in the polls, David Miliband has said.

The former foreign secretary, who lost to his brother in the party leadership election two years ago, said it would be a "tall order" to gain the seats in the South necessary for victory.

Mr Miliband intervened as his brother sought to deflect concern that he has an image problem and is too "geeky" to defeat David Cameron.

Ed Miliband will deliver his keynote speech to delegates at Labour's conference in Manchester today. Several of his senior colleagues have warned that many voters still do not know who he is.

Speaking on the fringes of the conference, David Miliband was careful to praise his brother for giving the party a chance of victory when many thought it would "fall apart". "We have put ourselves in a position to win the next election. That is a real achievement," the elder brother said. He said Ed Miliband and the shadow cabinet team had made Labour look "disciplined" when the Government seemed "ragged".

"Equally, they know and I know we have got a very big mountain to climb still. We have got 10 seats out of 212 in the south of England outside London. That is a very big, very tall order to turn over."

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He said his brother and Ed Balls were right to acknowledge that fiscal responsibility "is not a choice" but a "requirement to be a party of government".

However, the relationship between the two brothers is said still to be strained, two years after the divisive leadership election. The former foreign secretary remains popular among Blairites in the party but has so far resisted calls to return to the front bench.

Two of David Miliband's biggest backers, the New Labour architects Lord Mandelson and Alastair Campbell, issued their own warning to the party not to retreat to its "comfort zone".

Mr Campbell, the former spin doctor to Tony Blair who remains in regular contact with Ed Miliband, warned that the party was in danger of "complacency".

"Don't be positive and upbeat because we're 10 points ahead in the polls," Mr Campbell said. "We should be positive and upbeat if we are 20 points ahead in the polls with a month to go to the election."

Mr Campbell said Ed Miliband must now develop policies that engaged the public. "It is up to the Labour Party, Ed in particular, to show that there's an understanding of the modern world, there's a policy agenda," he said.

Lord Mandelson told delegates to be vigilant against the "attraction" of its "comfort zones and of complacency". He said Gordon Brown lost the last election because he did not look like a "natural PM" and drifted away from New Labour.

Jim Murphy, the shadow defence secretary, called for a referendum on Britain's membership of the European Union, saying "almost everyone" in the Labour Party would campaign for Britain to stay in the EU.

He told BBC Two's Daily Politics programme: "At some point, there will have to be a referendum ... I don't think it's for today or for the next year, but I think it should happen."